The present invention relates to controlled resistive braking of non-regenerative AC drives and more particularly to a resistive braking module with thermal protection.
Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as low frequency three-phase AC current. Despite being distributable efficiently, low frequency AC current is not suitable for end use in consuming facilities. Thus, prior to end use, power delivered by a utility is converted to a useable form. To this end, a typical power “conditioning” configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end-useable form (e.g., three phase, relatively high frequency AC voltage). A controller controls the inverter in a manner calculated to provide voltage waveforms required by the consuming facility.
Motors and linked loads are one type of common inductive load employed at many consuming facilities and, while the present invention is applicable to several different load types, in order to simplify this explanation an exemplary motor and load will be assumed. To drive a motor an inverter includes a plurality of switches that can be controlled to link and delink the positive and negative DC buses to motor supply lines. The linking-delinking sequence causes voltage pulses on the motor supply lines that together define alternating voltage waveforms. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. In an induction motor, the magnetic field induces a field in motor rotor windings. The rotor field is attracted to the rotating stator field and thus the rotor rotates within the stator core. In a permanent magnet motor, one or more magnets on the rotor are attracted to the rotating magnetic field.
One technique for stopping a motor and linked load is to cut off power to the inverter such that the stator field is eliminated. Without power the stator and rotor fields diminish and eventually the rotor slows and stops. While this stopping solution is suitable for some applications, this solution is unacceptable in other applications where motors have to be stopped relatively quickly for safety or duty cycle concerns.
A technique for actively slowing the motor involves using a resistive brake circuit. The resistive brake includes braking resistors coupled across the phases of the motor and switches for enabling the braking resistors. When the switches are closed, and the motor is isolated from the drive unit (i.e., the drive signals are isolated), the motor effectively acts as a generator to provide current to the load created by the braking resistors. Hence, the energy stored in the rotor and stator fields and the inertial energy stored in the rotating motor/load are transferred to the braking resistors. The power transferred to the braking resistors is dissipated as heat.
Because, the energy stored in the motor is dissipated as heat, the brake unit may overheat in situations where the duty cycle between motoring and braking is short and the brake is exercised repeatedly. Typical resistive braking units employ wire-wound resistors and depend on overheating the resistor wire to the point of failure as a thermal overload protection. However, even before the point of failure, the heat may build up to a sufficient level that the temperature of the unit exceeds the Underwriters Laboratory (UL) requirements for safe touch. Moreover, the failure mechanism of the wire-wound resistors limits the range of applications in which they may be used in terms of motor size and duty cycle.
Another technique for braking a rotating motor involves controlling the inverter that supplies the drive signals to the motor such that the drive signals lag the motor fields (i.e., typically the drive signals lead the motor fields to drive the motor). The motor acts as a generator in this situation, and the power generated thereby can be dissipated by the inverter as heat or transferred back to the DC bus in a regenerative fashion. This technique requires more complex inverter circuitry and control logic, thereby increasing cost. Additionally, if a motor and drive unit configured to support a non-braking application is instead to be used to support an application that requires braking, the entire drive unit would have to be changed to facilitate the braking feature.
Therefore, there is a need for a resistive braking system that can stop a load (e.g., motor and connected load) within a given time period that requires a relatively small and inexpensive brake mechanism that can be installed with an exiting equipment base and that will maintain operating temperatures within desired operating limits.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.